1. Field of the Invention
The present invention relates to a binder resin for a toner and also to a positively chargeable toner containing such a binder resin, wherein the toner is used for developing latent images in electrophotography, electrostatic recording, electrostatic printing, etc.
2. Discussion of the Related Art
As disclosed in U.S. Pat Nos. 2,297,691, 2,357,809, and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer and subsequently exposing the layer to eliminate the charge on the exposed portion, and visualizing the formed image by adhering a colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As indicated above, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
In general, in order to obtain a positively chargeable toner, an binder resin used therefor is a styrene-acrylic resin which can be easily positively charged. However, the styrene-acrylic resin has low mechanical properties, so that the resulting toner is less durable for continuous printing. Also, when a resin having a relatively high molecular weight is used in order to solve this problem, the fixing ability of the toner becomes extremely poor. Further, although a polyester resin having excellent mechanical properties can provide both good fixing ability and stability upon continuous printing, its negative chargeability is too strong, making it difficult to provide positive chargeability.
In order to solve the above problems, the following methods for blending polyester resins having excellent fixing ability with styrene-acrylic resins having a small change in the triboelectric charge under the high-temperature, high-humidity conditions when compared with that under normal-temperature, normal-humidity conditions have been known. For instance, examples of such methods include:
(1) Methods for blending polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open Nos. 49-6931, 54-114245, 57-70523, and 2-161464); PA0 (2) Methods for chemically linking polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open No. 56-116043); PA0 (3) Methods for copolymerizing unsaturated polyesters with vinyl monomers (see Japanese Patent Laid-Open Nos. 57-60339, 63-279265, 1-156759, and 2-5073); PA0 (4) Methods for copolymerizing polyester resins having a (meth)acryloyl group with vinyl monomers (see Japanese Patent Laid-Open No. 59-45453); PA0 (5) Methods for copolymerizing reactive polyesters with vinyl monomers in the presence of polyester resins (see Japanese Patent Laid-Open No. 2-29664); and PA0 (6) Methods for forming a block copolymer by linking polyester resins and vinyl resins with an ester bond (see Japanese Patent Laid-Open No. 2-881). PA0 (1) A binder resin for a toner comprising: PA0 (a) Controlling the average diameter by changing the compatibility of the vinyl resin component ini the resin (1) with the resin (2); and PA0 (b) Controlling the average diameter by changing the blending ratio of the resin (1) to the resin (2).
However, since the polyester resins have inherently poor compatibility with the styrene-acrylic resins, mere mechanical blending of the components may result in poor dispersion of the resins and the internal additives such as a carbon black at the time of production of the toner in certain blending ratios. This may in turn cause unevenness in the triboelectric charge of the toner, thereby causing problems such as background in the formed images. Further, when the two types of resins have different molecular weights, the differences in their melt viscosities are likely to take place, thereby making it difficult to make the grain size of the resin for the dispersed domain fine. In such a case, when a toner is produced with such resins, the dispersion of the internal additives such as a carbon black becomes extremely poor, so that such a problem arises that the resulting toner results in poor image quality. Moreover, in the case where the vinyl monomers are copolymerized with the reactive polyesters, it is applicable only in a restricted compositional range in order not to allow gelation to take place.